This invention relates to an electrical switching apparatus and, more specifically, to an electrical switching apparatus having a magnetic blow-open device.
Electrical switching apparatus for power distribution systems includes devices such as, but not limited to, circuit breakers, network protectors and transfer switches. Circuit breakers include both power circuit breakers and current limiting circuit breakers. A power circuit breaker is typically disposed upstream of other circuit breakers and is structured to delay its tripping action allow a limited fault current to pass therethrough. This allows the downstream circuit breaker closer to the fault to trip. If the fault current is not interrupted by a downstream circuit breaker, the power circuit breaker will trip. Power circuit breakers, however, are unable to withstand very high, e.g. greater than 150 kA, fault currents. For such higher ratings, a current limiting circuit breaker is used. A current limiting circuit breaker is structured to open within a few milliseconds after experiencing a very high fault current, thus limiting the fault current to a value much less than the available prospective peak value.
One type of current limiting circuit breaker is a xe2x80x9cfusedxe2x80x9d current limiting circuit breaker. A fused current limiting circuit breaker uses the current limiting action of series connected current limiting fusses to achieve the required rapid response. This allows the circuit breaker to open at a slower rate, i.e. similar to a power circuit breaker. It would be desirable to have a xe2x80x9cfuselessxe2x80x9d current limiting circuit breaker.
A common type of power air circuit breaker has a molded casing housing and multiple pole assemblies all driven by a common operating mechanism through a rotatable pole shaft. Each pole includes a contact arm carrier pivoted by a pole arm on the pole shaft between closed and open positions. The contact arm carriers support contact arms having movable contacts which engage stationary contacts with the contact arm carrier in the closed position. Typically, the contact arm is made up of a number of contact arm laminations or fingers mounted on a common pivot pin on the carrier. Contact springs bias the contact fingers against the stationary contacts in the closed position to provide contact pressure and to accommodate for wear. As the movable contact carrier is closed, typically by a spring operated closing and/or reset mechanism, the contact springs are compressed.
Typically, the switching apparatus is structured so that all of the movable contacts are held against the stationary contact by a mechanical force. The mechanical force is sufficient to overcome the electromagnetic force created by current flowing through the contacts and the force of the compressed contact springs. To open the circuit breaker, the mechanical force is released and the all of the contacts are separated. Separation of the contacts is caused by the contact springs expanding. As the contact springs expand, the movable contact carrier is moved away from the stationary contact. By the time the contact springs are fully expanded, the movable contact carrier has enough momentum to pull the contact arm fingers away from the stationary contact. When a large over-current is present, the separation of the contacts is also aided by the electromagnetic repulsion force between the stationary and movable contacts.
The prior art release mechanism included a rotatable pole shaft having a lobe that was coupled by a link assembly to the movable contact carrier. The link assembly included a rigid link member disposed between the pole shaft and the contact carrier. The combination of the pole shaft and the link assembly created the mechanical force that held the movable contact carrier in place. The release mechanism was structured so that the pole shaft rotated causing the lobe to move away from the stationary contact. As the lobe moved away from the stationary contact, the mechanical force was released and the contact springs moved the movable contact away from the stationary contact. The link assembly was rotatably attached to both the pole shaft and the movable contact. Thus, movement of the contact carrier was always associated with the rotation of the pole shaft. The link assembly was not structured to have a knee joint or otherwise be flexible.
From the perspective of a current limiting circuit breaker, the prior art release mechanism has several disadvantages. First, the release mechanism was generally structured to release all poles of the switching apparatus at one time. Second, the release mechanism did not respond rapidly to an over-current position. Additionally, power circuit breakers can not be easily converted into current limiting circuit breakers
There is a need, therefore, for a release mechanism that allows for each pole of a multi-pole switching apparatus to separate as soon as an over-current condition occurs in that pole.
There is a further need for a release mechanism having a rapid response time.
There is a further need for a release mechanism that can be integrated with existing switching apparatus. There is a further need for a release mechanism that can be incorporated into a power circuit breaker to convert the power circuit breaker into a current limiting circuit breaker.
This need and others are satisfied by the invention which is directed to electrical switching apparatus in which an electromagnetic blow-open device is incorporated into the assembly connecting the pole shaft to the movable contact.
The electromagnetic blow-open device includes a link assembly having a first link member and a second link assembly which are rotatably coupled to each other at a knee joint. The electromagnetic blow-open device also includes an actuating arm coupled to the fist and second link assemblies at the knee joint. The rotation of the first and second link assemblies relative to each other is limited by a stop means. Thus, when assembled, the first and second link assemblies may not rotate past a certain point. When the switching device is closed, or reset, the first and second links are brought to the point where no more rotation is possible. This is the over-toggle position of the link assembly. The link assembly is held in the over-toggle position by the force biasing the movable contact away from the stationary contact. This force includes both the electromagnetic repulsion force between the contacts, when current is flowing, and the force of the contact springs on the movable contact carrier.
The actuating arm includes a flapper assembly. The flapper assembly is, generally, a flat member of ferromagnetic material which extends adjacent to the contact arm fingers on the movable contact carrier. As a current passes through the contact arm fingers a magnetic field is created. The magnetic field attracts the flapper assembly. Under normal operating conditions, the magnetic field is not strong enough to overcome the force of the contact springs holding the link assembly in the over-toggle position. When an over-current condition occurs, the strength of the magnetic field increases and draws the flapper assembly towards the contact arm fingers. As the flapper is drawn toward the contact arm fingers, the actuating arm acts on the knee joint causing the first and second links to move back through the toggle position. Once the first and second links are out of the over-toggle position, the linkage will collapse and the movable contact carrier is free to move away from the stationary contact. Separation of the contacts is assisted by the force of the contact springs expanding and the magnetic field generated by the fault current.
Thus, when the link assembly moves out of the over-toggle position, the contact carrier moves away from the stationary contact without the pole shaft rotating. The time-current characteristics of the trip unit are coordinated with the tripping action of the blow-open device, so that, when a pole opens, the release mechanism for the remaining poles is actuated. Opening the circuit breaker with the poleshaft also acts to reset the blow-open device by moving the link assembly back into the over-toggle position. Resetting the release mechanism will maintain the link assembly in the over-toggle position even when the blow-open device has not been activated, e.g., after the circuit breaker is manually opened.
Using this electromagnetic blow-open device allows one to change a power circuit breaker into a current limiting circuit breaker simply by installing the electromagnetic blow-open device in place of the prior art link assembly.